1. Field of the Invention
The present invention relates, in general, to brushless DC motors and, more particularly, to a bearing system capable of allowing the gap between the shaft and bearing of a brushless DC motor to keep desired eccentricity when the shaft is operated at high speeds, thus maintaining a dynamic pressure at a position between the shaft and bearing and preventing a formation of an oil whirl during a high speed operation of the motor, and improving a motor's dynamic characteristics, such as low operational vibrations and noises. The present invention also relates to a brushless DC motor using such a bearing system.
2. Description of the Prior Art
As well known to those skilled in the art, small-sized precision motors, typically used in office machines, are required to be designed to rotate at high speeds and provide dynamic characteristics of low operational vibrations and noises in order to meet the necessity of high speed operation and provide a large capacity of such office machines. Therefore, it is a recent trend to change bearings for such motors from ball bearings into hydrosintered or hydrodynamic bearings with excellent dynamic characteristics.
FIG. 1 is a sectional view of a spindle motor using a conventional hydrodynamic bearing. As shown in the drawing, a bearing 1a is vertically and concentrically arranged on the base panel of a motor housing 1 through a fitting process, while a shaft 2 is rotatably and downwardly inserted into the bearing 1a. A core 1b, with a coil 1c, is arranged around the bearing 1a, thus forming a stator of the motor. The top end of the shaft 2 is coupled to a cap-shaped rotor 3, thus being rotatable along with the rotor 3. A cylindrical magnet 3a is attached to the inner surface of the rotor's sidewall, thus surrounding the stator 1b. When the motor is started, electric power is applied to the coil 1c of the core 1b, thus allowing the magnet 3a to generate magnetic force. The rotor 3 is thus rotated along with the shaft 2 at high speeds.
In an operation of the above spindle motor, the shaft 2 is rotated inside the bearing 1a at high speeds. In such a case, oil, filled in the gap between the bearing 1a and the shaft 2, generates a hydrodynamic pressure and effectively supports the shaft 2 in a radial direction during a high speed rotation of the shaft 2. When the shaft 2 is rotated at high speeds as described above, the rotor 3, carrying a disc (not shown) thereon, is rotated at high speeds, thus allowing data stored in the disc to be reproduced.
However, such a conventional hydrodynamic bearing for spindle motors is problematic in that it generates an oil whirl, thus being unstable during an operation of the motor. Such an oil whirl is formed as bearing eccentricity of the gap between the bearing 1a and the shaft 2 is gradually reduced at a speed higher than a predetermined level. That is, an increase in the rotating speed of the shaft 2 causes the Sommer Felt number to be reduced, thus gradually reducing eccentricity of the shaft 2 in the bearing 1a. Such a reduction in the eccentricity of the shaft 2 is caused when the oil, rotated along the shaft 2, has a given speed distribution. Such a reduction in eccentricity is typically formed in genuine circular hydrodynamic bearings free from dynamic pressure grooves. Therefore, the conventional hydrodynamic bearings reduce a motor's dynamic characteristics, such as low operational vibrations and noises, during a high speed operation of the motor.
In an effort to overcome the above problems experienced in the above spindle motors, Japanese Patent Laid-open Publication No. Hei. 7-110,028 discloses a hydrodynamic bearing. However, the above Japanese patent is problematic in that it is very difficult to form the dynamic pressure grooves on both the shaft and the bearing surface. Another disadvantage experienced in the above Japanese bearing is that the bearing reduces work efficiency while assembling the shaft with the bearing.